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In too many ways, court observers say, it was a prosecutor’s nightmare: a morally reprehensible key witness, a possible misstep by police, and too little else to make up for the shortcomings, resulting in the implosion Thursday of the state’s case against the accused killers of four people in Mattapan.

A jury’s decision to acquit Edward Washington and its failure to reach a verdict for his codefendant, Dwayne Moore, sent shockwaves through the legal community, which had closely watched the case.

The loss was a setback for District Attorney Daniel F. Conley, who been under immense public pressure to swiftly and successfully prosecute those accused of killing four victims, including a mother and her 2-year-old son, after an armed home invasion seeking cash and drugs in 2010.

Jake Wark, a spokesman for Conley, said the mood was somber in his office after the announcement of the jury’s findings, “as it is after every homicide trial, no matter what the verdict might be.’’

Thursday, Conley vowed to retry Moore, with a new trial date expected to be scheduled next week. But the opportunity to prosecute Washington is over, because authorities are legally prohibited from retrying someone who has been found not guilty, leaving only questions about what might have been done differently.

In interviews, criminal defense lawyers and former prosecutors said Conley might have waited to bring the case to trial. There is no statute of limitations for murder, meaning there was no official deadline against which he was working. With additional time, new witnesses might have come forward, evidence might have been unearthed, or scientific investigative techniques might have been discovered. But waiting carried risks of faded memories, lawyers said. Conley also faced heavy pressure to bring the high-profile case to trial quickly so that victims’ families could see justice done.

Most problematic for the prosecution was its reliance on Kimani Washington as a crucial witness, observers said. The 36-year-old career criminal admitted that he participated in the robbery, but said he left before the shootings began.

“The pivotal witness was clearly seen by the jury as being at minimum not credible and at maximum menacing,’’ said Martin Weinberg, a criminal defense lawyer.

During cross-examination by defense lawyers, Kimani Washingtonadmitted that he abided by the street code of lying to police and acknowledged that he had agreed to testify against Washington, his cousin, and Moore, in exchange for a reduced sentence of 16 to 18 years in prison.

The witness admitted he had changed his story several times during interviews with police. He was also questioned about his five children by different women, his drinking and his work as a pimp, the latter was likely to be particularly troubling for jurors as they assessed his credibility as a witness, observers said.

“When you’re on the stand and admit to being a pimp, that turns everyone off,’’ said Bob Griffin, a criminal defense lawyer and former prosecutor. “With a drug dealer or a drug addict, they lie, but you can usually squeeze the truth out of them. But someone who is as cold-blooded as a pimp is not someone you want to believe in.’’

“One has to wonder whether the jury struggled over the credibility and motivations of Kimani Washington and if the Commonwealth made too much of a deal with the so-called devil,’’ said Brad Bailey, a criminal defense lawyer who previously worked as a state and federal prosecutor.

Yet, flawed as Washington may have been, observers said prosecutors probably had little choice but to put him on the stand.

“There’s an old theory on the prosecution side that it takes a rat to catch a rat,’’ Bailey said. “You take what you can get, and in this instance, they felt the tradeoff of 16 years for Kimani Washington, who was facing life in prison, was a deal they had to make.’’

Observers said prosecutors also grappled with a police omission, namely that investigators did not test Kimani Washington’s hands for gunshot residue when they took him in for questioning in the early hours of Sept. 28, 2010.

Police did not directly respond to questions about the gunshot residue testing, referring instead to a statement released byPolice Commissioner Edward Davis saying, “We continue to extend our compassion and sorrow to the family and friends of the victims, as well as express our steadfast dedication to pursue the prosecution of Dwayne Moore.’’

Some observers said the decisions rendered by the jury should be viewed not as a loss for the prosecution, but as justice served.

“Probably nothing went wrong; what went right is a jury took a good hard look at sketchy evidence,’’ said Jeffrey Denner, a criminal defense lawyer.